1. Field of the Invention
The energy crisis of recent years has stimulated research in the field of alternate and hybrid fuels. One area of particular interest relates to fuels for commercial and agricultural vehicles, which are typically powered by diesel engines. The prospect of farmers becoming self-sufficient in regard to their energy needs has led to investigations of vegetable oils as diesel fuel substitutes. Deterrents to this concept are the generally inferior fuel properties of crude vegetable oils as compared to those of diesel oil. Of particular concern is the inherently high viscosity which causes poor atomization in direct-injected diesel engines. This results in fouling of the injectors and cylinders as well as a buildup of noncombusted fuel in the crankcase causing a thickening of the lubricating oil. This invention relates to a blended vegetable oil fuel which circumvents these problems.
2. Description of the Prior Art
One approach to the utilization of vegetable oil as fuel has been to mix it with conventional diesel oil. Insofar as these blends must contain at least two-thirds diesel fuel in order to have acceptable properties, they fall short of meeting the farmer's goal of energy self-sufficiency. Cracking and refining are effective in upgrading vegetable oils, but add considerably to the expense and also negate direct on-the-farm utilization of the harvested product. Likewise, transesterification with a lower alcohol yields a fuel with lower viscosity and acceptable performance properties, but reduces the feasibility of direct use. Moreover, the transesters have a solidification temperature of about 4.degree. C., requiring the use of fuel preheaters in colder climates.
The concept of diluting the vegetable oil with ethanol, another energy source being investigated for on-the-farm generation, is confronted with the same difficulties characteristic of diesel fuel-ethanol hybrids. As pointed out by Wrage et al. [Technical Feasibility of Diesohol, ASAE Paper No. 79-1052 (1979)], the most critical problem is phase separation. Anhydrous ethanol and No. 2 diesel oil are miscible at room temperature, but trace amounts of water in the mixture will cause a phase separation and movement of the ethanol and water to the top of the container. The water tolerance of blends decreases with decreasing temperature. At 0.degree. C., a water concentration of only 0.05% will cause phase separation. Since this amount can readily be absorbed in the fuel during transport and storage, anhydrous ethanol-oil blends tend to be impractical.
Accordingly, a preponderance of the research efforts on hybrid fuels has been aimed at increasing the water tolerance to not only allow for water absorption, but also to permit the use of aqueous alcohol. As opposed to anhydrous alcohol, the aqueous form having at least 5% water content is within the production capabilities of on-farm stills. Also, its recovery requires substantially less energy, and it is therefore less costly to produce. Moreover, it has been reported that when water is properly incorporated into a diesel fuel, it serves as a heat sink, thereby lowering combustion temperatures and reducing NO.sub.x and smoke emissions [G. Gillberg et al., Microemulsions as Diesel Fuels, pp. 221-231 in J. T. Zung (ed.), Evaporation-Combustion of Fuels. Advances in Chemistry Series No. 166, ACS]. This phenomenon is also discussed by N. R. Iammartino [Chem. Eng. 24: 84-88 (Nov. 11, 1974)], D. W. Brownawell et al., U.S. Pat. No. 3,527,581, and E. C. Wenzel et al., U.S. Pat. No. 4,083,698.
The intimate admixture of water and oil results in either a macroemulsion or a microemulsion. Macroemulsions have dispersed particles with diameters in the 200 to 10,000 nm. range and are not stable, eventually separating into two phases. Microemulsions are transparent, thermodynamically stable colloidal dispersions in which the diameter of the dispersed-phase particles is less than one-fourth the wavelength of visible light. Considerably more surfactant is required to create a microemulsion than a macroemulsion since the volume of the interphase of a microemulsion is an appreciable percentage of the total volume of the dispersed sphere (the core plus the interphase). Microemulsions of aqueous ethanol in vegetable oils are generally accepted as micellar systems and may be classified as detergent or detergentless.
In U.S. Pat. No. 4,083,698, Wenzel et al. prepares stable water-in-oil emulsions comprising (a) a hydrocarbon fuel, (b) water, (c) an alcohol, and (d) a multicomponent surfactant system comprising: (1) a long-chain fatty acid salt, or, more preferably, an ammonium or sodium long-chain fatty acid salt, or mixture thereof as the detergent; (2) a free unsaturated long-chain fatty acid, or a mixture of a free unsaturated organic acid and a free saturated long-chain fatty acid; and (3) a nonionic surfactant typified by ethylene oxide condensation products and esterification products of a fatty acid with ethylene oxide. Weeks (U.S. Pat. No. 2,892,694) prepares a water-emulsified motor fuel by means of a detergent-type emulsifier comprising the reaction product of alkyl-4-sulfophthalate and ammonia or an amine.
In the commonly assigned application Ser. No. 06/256,206, A. W. Schwab discloses stabilizing a diesel fuel microemulsion having relatively high levels of water and alcohol by means of a two-component surfactant system. One of the components is N,N-dimethylethanolamine which functions as the detergent, and the other is a long-chain fatty acid substance. Increasing levels of surfactant as necessitated by the higher levels of ethanol and water has the effect of increasing the fuel's viscosity.